/Engine/Source/Runtime/Slate/Public/Widgets/Layout/SMenuOwner.h
For menu bar or sub-menu widgets, returns the currently open menu, if there is one open
Menu anchor, or null pointer
[TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< const [SMenuAnchor](API\Runtime\Slate\Widgets\Input\SMenuAnchor) > GetOpenMenu() const